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The Powder River Basin occupies a 22,000-mi 2 (35,000-km 2 ) area of northeastern Wyoming and southeastern Montana. Rocks of the Tullock Member (lower Paleocene) of the Fort Union Formation represent alluvial sedimentation in a Laramide intracratonic basin. The Cretaceous-Tertiary time line at or near the base of the Tullock provides temporal control for the timing and distribution of these basin infilling events. Tullock sediments have a maximum thickness of about 113 m (371 ft) in the north and 439 m (1,440 ft) in the south. The infilling Tullock clastic sediments merged eastward with distant delta-plain systems containing aggrading distributary channels that flowed into the Cannonball epicontinental sea.The Tullock Member was deposited in a continental fluvial environment. Channel deposits (comprising about onethird of the sequence) contain mainly trough and tabular planar crossbedded and climbing-ripple laminated sandstone; reactivation surfaces, liquefaction fronts, and structures resulting from soft-sediment deformation are also common, suggesting episodic rapid deposition, saturation of sediments, and a high watertable. Fine-grained overbank deposits (making up about two-thirds of the sequence) show color mottling, contain plant, wood, and coal fragments, have obscure bedding or no apparent internal structure, and contain thin coal beds.Tullock sandstone geometry and sedimentary structures suggest bankfull depths equal to paleochannel thickness, low fluvial gradient, low sinuosity, highly stable vegetated banks, and lenticular channel form. Channel systems carried predominantly suspended loads on mature low-gradient flood plains. Mud-dominated sediment moved through Tullock fluvial channels principally by downstream aggradation by bar construction, and build-up of the floodplain over low levees by overbank flooding, depositing mainly suspended material.Sedimentary structures and geometry of Tullock sediments suggest deposition in anastomosed river systems.

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